blood in test tubes

Don Bartletti/Los Angeles Times

Scientists believe red blood cells' flexibility contributes to their ability to circulate for an average of 120 days. Above, viles of blood are prepared to be screened at the American Red Cross National Testing Laboratory in San Diego.

Scientists believe red blood cells' flexibility contributes to their ability to circulate for an average of 120 days. Above, viles of blood are prepared to be screened at the American Red Cross National Testing Laboratory in San Diego. (Don Bartletti/Los Angeles Times)

Scientists believe red blood cells' flexibility contributes to their ability to circulate for an average of 120 days. Above, viles of blood are prepared to be screened at the American Red Cross National Testing Laboratory in San Diego.